In many situations it is desirable to combine a light source, such as an LED, within the same package as an electronic circuit. For example, it has been found that Gallium Arsenide (GaAs) Field Effect Transistors (FETs) have better switching performance when they are subjected to high intensity light during the switching operation. This enhanced performance stems from a decrease in the “slow tail effect” when the FET is operated in the presence of light. U.S. Pat. No. 5,808,322, issued Sep. 15, 1998, illustrates a high intensity light integrated with a GaAs FET switch for use in microwave test equipment. U.S. Pat. No. 6,876,271, issued Apr. 5, 2005, illustrates a light source mounted within the same chamber as a FET for use as a damping circuit. Both of these patents are hereby incorporated by reference herein.
In the U.S. Pat. No. 5,808,322, the LEDs are mounted to a small pc board that gets mounted to the back side of a carrier pc board. The carrier pc board has holes strategically placed to allow LED light to shine through the holes to the top side of the carrier pc board where another pc board is mounted upside down so that the switches are placed right over the holes. Thus light shines on the GaAs FET attenuator switches through the carrier pc board holes, directly impacting the FETs, thereby imparting increased performance (by way of faster switching times) to the FETs.
In the U.S. Pat. No. 6,876,271, the LED is mounted on a circuit board and is placed above the FET so that the light from the LED will directly impact the FET, thereby imparting increased performance (by way of faster switching times) to the FET.
An older alternate arrangement, for example the arrangement shown in Agilent Part Number 5087-7137, has demonstrated that LEDs can be mounted on the same plane with the FET switch within a metal microcircuit enclosure having a metal lid. The light from the LED (which is directed upward and away from the FET) is reflected from the inside of the top of the metal enclosure and impacts the FET. Such an arrangement works well, but requires a metal housing to achieve the proper light transfer. In addition, such packages have used unpackaged LED die for mounting within the enclosure. Metal enclosures are large and expensive to manufacture and in many situations it is desirable to use smaller ceramic and/or plastic enclosures.
For example, one packaging arrangement called a Quad Flat No Lead (QFN) package, is essentially a conductive lead frame on which is mounted the electronic circuitry. Such packages are manufactured in high volumes and at relatively low cost and size as compared to traditional metal and/or ceramic housings. An opaque (usually black) encapsulating material is placed around the electronic elements to fill out the volume within the enclosure. This material, in addition to providing support and stability to the electronic devices, acts to inhibit light transfer from the light source to the other elements, thus preventing the use of low cost non-metallic housings for applications that depend upon good light transfer for proper operation.